9.8. Prefetch abort handler

If the system contains no MMU, the Prefetch Abort handler
can simply report the error and quit. Otherwise the address that
caused the abort must be restored into physical memory. lr_ABT points
to the instruction at the address following the one that caused the
abort, so the address to be restored is at lr_ABT – 4. The virtual
memory fault for that address can be dealt with and the instruction
fetch retried. The handler should therefore return to the same instruction
rather than the following one.